Feature/topological sort

[frejknutarlewander]

When the model is closed, input-to-propagation now:

1. Partitions the invariant graph into layers.
2. Merges subsequent layers containing no dynamic cycles.
3. Topologically sorts each layer containing no dynamic cycle

During input-to-output search:

1. Propagation is performed in layer order (Starting at the first layer).
2. Before propagating layer n, if n contains one or more dynamic cycles, then n is topologically ordered during the search procedure.